1. Field of the Invention
The present invention relates to a fixing technology, more particularly, a heat plate and semicircular heating or heat-generating element which are effectively used for fixing a toner image. Also, the present invention relates to a belt type fixing device. Especially, the present invention relates to a heat plate for fixation having high reliability and durability and capable of increasing a temperature at a high rate, used for fixing a toner image in an image forming apparatus such as a copier, printer and so forth, which is based on an electrophotographic system; a heat plate for fixing a toner image having a temperature control property, the heat plate being of a temperature control type by which an upper limit of the temperature provided by a heating resistor layer can be set and controlled; a heat plate for fixing a toner image, the heat plate being of a self-temperature-control type by which a predetermined temperature distribution can be automatically set and controlled in a moment; a heat plate for fixing a toner image, the heat plate being of a complete self-temperature-control type having all the above functions; a semicircular heating member for fixing a toner image, the semicircular heating member being formed into a semicircle and curved by means of press forming; and a belt type fixing device into which the above components are incorporated, the belt type fixing device characterized in that energy can be saved and heating can be quickly conducted. Also, the present invention relates to various image forming apparatuses provided with the belt type fixing device of the present invention.
2. Description of the Related Art
In the electrophotographic system, which is widely used in copiers, printers and printing machines, generally positive or negative uniform electrostatic charges are applied onto a surface of a photoconductive insulator such as a photoreceptor drum. After the completion of this uniform electric charging step, when the photoconductive insulator is irradiated with a light image by various means, the electrostatic charges on the insulator are partially erased so that an electrostatic latent image can be formed. For example, when the photoconductive insulator is irradiated with a laser beams, surface electric charges in a specific portion can be erased. Due to the foregoing, an electrostatic latent image corresponding to image information can be formed on the photoconductive insulator. Next, fine powders of a developing agent, which is called a toner, are attached to the latent image portion on the photoconductive insulator in which electrostatic charges still remain. In this way, the latent image can be visualized. Finally, in order to obtain a print from the thus obtained toner image, the toner image is electrostatically transferred onto a recording medium such as recording paper. Finally, the thus transferred toner image is fused and fixed by applying heat, light or pressure.
As is well known, the fixing process for fixing a toner image, which is a final process of the electrophotographic system, is executed by using various fixing devices. At present, the following two types of fixing devices are commonly used. One is a flash fixing device for fusing and fixing toner by irradiating light such as a flash of light, and the other is a heat roller fixing device for fusing and fixing toner while applying pressure to toner by a heated fixing roller which is also referred to as a heat roller.
In general, in the heat roller fixing device, a heat roller and pressure roller are arranged opposed to each other, and a recording paper having a toner image transferred thereon is made to pass between a pair of rollers. The toner image is thermally fused and fixed onto the recording paper when the toner image is heated and pressurized at the same time. The practically used prior art heat roller comprises a light emitting heating tube such as a halogen lamp housed in a metallic pipe made of aluminum or stainless steel. FIG. 1 is a cross-sectional view showing a heat roller type fixing device composed as described above. The fixing device is provided with a heat roller 61 capable of rotating in the direction of arrow xe2x80x9caxe2x80x9d and a pressure roller 62 capable of rotating in the direction of arrow xe2x80x9cbxe2x80x9d. In the heat roller 61, there is provided a halogen lamp 63. On the other hand, a sheet of recording paper 38, onto the surface of which the toner image 51 has already been transferred in the previous process, is conveyed in the direction of arrow xe2x80x9ccxe2x80x9d and is inserted into a nip formed between the heat roller 61 and the pressure roller 62. Therefore, when the sheet of recording paper 38 passes through the nip between the heat roller 61 and the pressure roller 62, heat and pressure are simultaneously applied to the sheet of recording paper 38, so that the fused toner image can be fixed onto the surface of the sheet of recording paper 38.
However, the above heat roller utilizes radiation heat so as to heat the sheet of recording paper. Therefore, the following problems may be encountered. The heating efficiency is so low that it takes several minutes to several tens of minutes to heat the sheet of recording paper to a predetermined temperature necessary for thermal fixing, for example, it takes several minutes to several tens of minutes to heat the sheet of recording paper to 160xc2x0 C. Especially, when a copier, which is in a stopped state out of operation, is turned on, it takes time to start the copier and an operator must wait for a long period of time, which lowers the working efficiency.
Recently, a copier or printer has been linked with another electronic device such as a personal computer. Therefore, if an input signal is transmitted to the personal computer during the waiting time, the system does not proceed at once, because it takes time to raise the heat roller temperature. As a result, the processing rate of the entire system is decreased. That is, no matter how the processing rate of the electronic device may be increased, it is difficult to increase the processing rate of the entire system unless drastic measures are taken for increasing the rate of heating the toner fixing section. In order to solve the above problems, it is conventional that the heat roller is energized and heated even while the entire system is waiting for a successive operation. In this connection, about one hundred million electrophotographic devices are operated all over the world. Therefore, when consideration is given to the fact that all of the electrophotographic devices are energized while they are waiting for a successive operation, it is necessary to take countermeasures for saving energy which is consumed in energizing the heat rollers during the waiting time.
As one countermeasure for saving energy, concerning the heating system for heating the fixing device, instead of the halogen lamp system which is used at present, a direct heating system, induction heating system and others have been developed. Concerning the heating efficiency, when the heating efficiency of the direct heating system is 1, the heating efficiency of the induction heating system is 0.8, and the heating efficiency of the halogen lamp heating system is 0.6. According to a revision of the Law of Saving Energy of April in 1999, competition for saving energy has grown more intense while targeting the conformity to Mode Restriction issued by the Ministry of Trade and Industry in 2000 and also targeting the conformity to the Law of Top Runner in 2006. Equipment to satisfy the above laws on saving energy has already been coming into the market while they use their own techniques to save energy. Digital technique has been widely adopted and color images are processed by every image processing device, and further two or more devices are combined with each other so as to enhance the performance. In order to meet demand for increasing the processing rate, enhancing the image quality and saving energy, belt type fixing devices tend to be adopted.
For example, as shown in FIG. 2, the belt type fixing device includes: a fixing roller (having no heat source therein) 61 capable of rotating in the direction of arrow xe2x80x9caxe2x80x9d; a heating unit 65; an endless belt 66 provided between the fixing roller 61 and the heating unit 65; and a pressure roller 62 capable of rotating in the direction of arrow xe2x80x9cbxe2x80x9d, the pressure roller 62 coming into pressure contact with the fixing roller 61 via the belt 66. The heating unit 65 is formed into a roller arranged in parallel with the fixing roller 61. The heating unit 65 is composed of a cylindrical heat conductive roller 67 and a heating sheet 68 bonded inside the heat conductive roller 67. The heating sheet 68 includes a heating resistor capable of heating when an electric current flows into it. Heat generated in the heating sheet 68 is transmitted to the belt 66 via the heat conductive roller 67 surrounding the heating sheet 68. On the other hand, a sheet of recording paper 38, on the surface of which a toner image 51 is formed in the previous process, is conveyed in the direction of arrow xe2x80x9ccxe2x80x9d and inserted into the nip formed between the belt 66, which is guided on a surface of the fixing roller 61, and the pressure roller 62. Due to the foregoing, when the sheet of recording paper 38 passes through the nip formed between the rollers, heat and pressure are simultaneously applied to the sheet of recording paper 38. Therefore, the fused toner image can be fixed onto the surface of the sheet of recording paper 38.
When importance is attached to improvement of energy saving properties, the most important matter is how to reduce heat radiation from the endless belt for fixing, the fixing roller and the pressure roller. In order to save energy and reduce the temperature rising time and further in order to reduce the manufacturing cost of the device, the most important issue is how to reduce the heat capacity and the size of the device.
Recently, copiers have made rapid progress as follows. Recent copiers are provided not only with a simple function but also a multiple function, that is, the copiers are composed into compound machines having multiple functions of copying, printing and communicating (function of a facsimile device). Further, the copiers can process not only a monochrome image but also a full color image. In these copiers, various sheets of recording paper, the sizes of which are A5, B5, A4, B4 and A3, are used. When sheets of recording paper of small sizes such as A5 or B5 are frequently used, the surface temperature of a sheet threading portion of the heating member in the sheet threading section is decreased. In contrast, the surface temperature of a portion of the heating member in which the sheet of paper does not thread is increased. Therefore, there is a great difference in the surface temperature distribution. In the case of a conventional fixing device in which a halogen lamp is used, this difference in the surface temperature of the sheet threading portion and that of the portion in which the sheet does not thread is so great that a temperature boundary appears on the sheet of recording paper. Especially in the case of equipment in which processing is conducted at a high processing rate, for example, in the case of a color image forming apparatus, this phenomenon is remarkably caused, and the difference in the surface temperature distribution appears directly on a sheet of recording paper.
In the case of forming a monochrome image, no problems are caused. However, in the case of a fixing device used for forming a color image, there is a demand of having both the thermal fixing function and the full coloring function. In order to meet this demand, it is necessary to raise the temperature to 160xc2x0 C., which is the fixing temperature, in a temperature raising time of not more than 10 seconds, and it is also necessary for the surface temperature distribution of the heating member to be uniform in a range of fluctuation of xc2x15% while maintaining a high degree of energy saving. In the age of digital technology, various targets of developing the fixing device are set such as a reduction in electric power consumption, shortening of the temperature rising time and a technique for automatically controlling the surface temperature distribution. Especially in the case of equipment in which color images are processed, in order to obtain images of high quality, it is necessary to quickly raise the temperature and for the distribution of surface temperature to be uniform.
The present inventors have devised a direct heating system in which heating is directly conducted by energizing a heating resistor layer instead of a heating system in which heating is conducted by a light emitting heating tube such as a halogen lamp and others. That is, a heating resistor layer is formed on the inner circumferential face or the outer circumferential face of a metallic material tube via an electric insulating layer. When this heating resistor layer is energized and heated, the metallic material tube can be quickly heated. A fixing roller having this heating function capable of quick heating can be referred to as a quick heat roller (QHR). The present inventors conducted investigations into the performance of this quick heat roller, and as a result, it was found and proved that the quick heat roller can heat several times quicker than a halogen lamp system. Accordingly, it is possible to apply this quick heat roller to the fixing device of an electrophotographic apparatus such as a copier, printer and so forth.
Although the temperature raising performance of the quick heat roller is high, problems to be solved still remain in the method of forming the quick heat roller. Also, problems to be solved still remain in the field of reliability and durability.
For example, in the case of forming the heating resistor layer on the outer circumferential face of a metallic material tube, the forming method is relatively simple. Layers of various materials may be formed on the outer circumferential face by a spraying or dipping method. However, in this case, there are problems in the durability of the heating resistor layer. In the course of operation, the outer circumferential face of the quick heat roller repeatedly comes into contact with a recording paper, a thermistor for detecting temperatures and a toner separating pawl. Due to the above mechanical contact, each layer formed on the heating member is always worn out. Therefore, the life of the quick heat roller is impaired. When abrasion extends to the heating resistor layer, there is a possibility of electric leakage and burning out.
In the case of forming the heating resistor layer on the inner circumferential face of the metallic material tube, the heating resistor layer does not come into contact with a recording paper. Therefore, it is possible to ensure long life of the quick heat roller. However, it is very difficult to uniformly form the heating resistor layer on the inner circumferential face of the metallic material tube. Accordingly, the present inventors have devised a method in which each layer is formed on an insulating sheet so as to make a heating resistor sheet first and then the thus made heating resistor sheet is bonded on the inner circumferential face of the metallic material tube. However, according to the above method, it is not easy to ensure satisfactory mass production characteristics while uniformity of bonding, mechanical strength and reliability are being guaranteed.
Accordingly, the first object of the present invention is to provide a heat plate for fixation utilizing a heating resistor capable of being easily formed, the reliability of which can be guaranteed and the heating efficiency of which is high, and characterized in that the temperature can be quickly raised, energy can be saved, and the manufacturing cost can be reduced.
The second object of the present invention is to provide a heating member for fixation, the upper limit of temperature obtained by the heating resistor of which is automatically set so as to prevent the heating member from being overheated and burned out, and characterized in that the durability and reliability are high.
The third object of the present invention is to provide a multifunctional heating member of the self-temperature-control type or the complete self-temperature-control type capable of automatically controlling the surface temperature distribution of the heating member to a predetermined temperature.
The fourth object of the present invention is to provide a very small and multifunctional belt type fixing device using the above heating member, the manufacturing cost of which is low, capable of heating at a high heating rate and capable of greatly saving energy.
The fifth object of the present invention is to provide a compact and high-performance image forming apparatus in which the belt type fixing device of the present invention is used. Especially, the fifth object of the present invention is to provide an image forming apparatus based on an electrophotographic system.
These objects and other objects of the present invention will become more apparent in the following detailed descriptions.
In one aspect of the present invention, there is provided a heat plate for fixation comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least, an electric insulating layer and heating resistor layer in this order on the metallic base plate, wherein the heat plate is capable of raising the temperature of the metallic base plate to a fixing temperature when the heating resistor layer is energized and heated.
The heat plate for fixation of the present invention can be advantageously executed in various embodiments.
For example, it is preferable that the heating resistor layer of the heat plate be made of a mixture having added thereto a synthetic resin or glass capable of forming a matrix with Mo or Ag as a metallic resistor material.
It is also preferable that the electric insulating layer of the heat plate be a heat transmission strengthening layer consisting of an electric insulating material having a high coefficient of thermal conductivity, and when the heating resistor layer is energized and heated via the heat transmission strengthening layer, the generated heat is effectively conducted onto the metallic base plate.
Further, it is preferable that the heating resistor of the heat plate comprise a laminar heating resistor which is composed of a heat transmission strengthening layer, heating resistor layer, heat insulation strengthening layer, heat reflecting layer for reflecting heat rays onto the metallic base plate side and a protective layer laminated in this order on the metallic base plate.
Furthermore, it is preferable that the metallic base plate of the heat plate be composed of one of an aluminum plate, stainless steel plate, common steel plate and galvanized sheet iron, and a reverse side of the metallic base plate, in contact with the heating resistor layer has a roughened surface.
Furthermore, it is preferable that the film thickness of the heating resistor layer of the heat plate in the portion of the lead-in terminal side be smaller than that in the central portion of the heating resistor layer, so that heating electric power on the lead-in terminal side is higher than that of the central portion, and thus the surface temperature distribution of the metallic base plate becomes uniform.
Furthermore, it is preferable that each layer composing the heating resistor of the heat plate be formed by the screen printing method, and the film thickness of each layer be controlled by applying multiple layer printing, and after the layers have been formed, the resulting layered structure heated and pressurized in order to make the film of each layer tight and stabilize the resistance of each layer, and further subjected to the energizing and heating treatment for aging so as to greatly enhance the reliability and durability of each layer.
In another aspect of the present invention, there is provided a temperature controlling type heat plate for fixation comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least a heat transmission strengthening layer, heating resistor layer for controlling temperature, heat transmission strengthening layer, heating resistor layer, heat insulation strengthening layer, heat reflecting layer and protective layer in this order on the metallic base plate, wherein the heating resistor layer for controlling temperature is made of a resistor material having a positive temperature coefficient, and when the heating resistor is energized and heated, the temperature of the heating resistor is automatically set and controlled so that the temperature is not increased to not less than 300xc2x0 C.
In this heat plate, it is preferable that the temperature controlling type heating resistor layer having a positive temperature coefficient be made of ultra fine powders of Mo or Ag as a primary resistor material, and at least one of the single metals of Ge, Si, Sn and Zn, SnZn alloy, or Y3FeO12, as an auxiliary resistor material.
Further, in another aspect of the present invention, there is provided a self-temperature-controlling type heat plate for fixation comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least, a heat transmission strengthening layer, heating resistor layer for controlling temperature, heat transmission strengthening layer, heating resistor layer, heat insulation strengthening layer, heat reflecting layer and protective layer in this order on the metallic base plate, wherein the heating resistor layer for controlling temperature is made of a resistor material having a negative temperature coefficient, and when the heating resistor layer is energized and heated by a constant current power source, a low temperature section is heated by high electric power because of its high resistance, and a high temperature section is heated by low electric power because of its low resistance, so that a surface temperature obtained by the heating resistor layer is compensated by the thus generated heat and a local temperature distribution of the heat plate can be automatically controlled in a moment to be uniform.
In this heat plate, it is preferable that the temperature controlling type heating resistor layer having a negative temperature coefficient be made of as a primary resistor material, ultra fine powders of Mo or Ag and as an auxiliary resistor material, a semiconductor material comprising a metal of Group III or V on the Periodic Table added to a metal of Group IV, or powders of lower oxide of transition metal.
Further, in still another aspect of the present invention, there is provided a complete self-temperature-controlling type heat plate for fixation comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least, a heat transmission strengthening layer and a second temperature controlling type heating resistor layer on a set of a heat transmission strengthening layer and a first temperature controlling heating resistor layer, and further laminating thereon a heat insulation strengthening layer, heat reflecting layer and protective layer in this order.
In this heat plate, it is preferable that the second temperature controlling heating resistor layer have a positive temperature coefficient and its temperature is controlled by a common electric power source, and the first temperature controlling heating resistor layer have a negative temperature coefficient and its temperature controlled by a constant current electric power source.
In addition to the above-described heat plates for fixation, the present invention provides a semicircular heating member for fixation in which the above heat plate is curved so that the metallic base plate has a convex surface. Although no description is included here, in order to avoid duplication, the heat plate used for the heating member of the present invention includes various embodiments as described above, and will be explained in detail below.
Further, the present invention provides a multifunction fixing roller-circumscribing belt type fixing device comprising a pressure roller arranged inside a semicircular heating member for fixation, an endless belt for fixation wound round an outer circumference of the semicircular heating member, and a fixing roller arranged on a face of the endless belt opposing to the pressure roller, in which the endless belt is heated to a predetermined temperature, and while a load is acting between the pressure roller and the fixing roller, a recording sheet preliminarily heated by a heat plate for preliminarily heating is passed between the pressure roller and the fixing roller, thereby thermally fixing a toner image on the recording sheet, and
the semicircular heating member is one member selected from the group consisting of:
(1) a semicircular heating member for fixation which comprises a heat plate for fixation comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least, an electric insulating layer and heating resistor layer in this order on the metallic base plate, wherein the heat plate is capable of raising the temperature of the metallic base plate to a fixing temperature when the heating resistor layer is energized and heated, and the heat plate is curved so that the metallic base plate has a convex surface,
(2) a semicircular heating member for fixation which comprises a temperature controlling type heat plate comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least, a heat transmission strengthening layer, heating resistor layer for controlling temperature, heat transmission strengthening layer, heating resistor layer, heat insulation strengthening layer, heat reflecting layer and protective layer in this order on the metallic base plate, wherein the heating resistor layer for controlling temperature is made of a resistor material having a positive temperature coefficient, and when the heating resistor is energized and heated, the temperature of the heating resistor is automatically set and controlled so that the temperature is not increased to 300xc2x0 C. or more, and the heat plate is curved so that the metallic base plate has a convex surface,
(3) a semicircular heating member for fixation which comprises a self-temperature-controlling type heat plate comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least, a heat transmission strengthening layer, heating resistor layer for controlling temperature, heat transmission strengthening layer, heating resistor layer, heat insulation strengthening layer, heat reflecting layer and protective layer in this order on the metallic base plate, wherein the heating resistor layer for controlling temperature is made of a resistor material having a negative temperature coefficient, and when the heating resistor layer is energized and heated by a constant current power source, a low temperature section is heated by high electric power because of its high resistance, and a high temperature section is heated by low electric power because of its low resistance, so that a surface temperature obtained by the heating resistor layer is compensated by the thus generated heat and a local temperature distribution of the heat plate can be automatically controlled in a moment to be uniform, and the heat plate is curved so that the metallic base plate has a convex surface, and
(4) a semicircular heating member for fixation which comprises a complete self-temperature-controlling type heat plate comprising: a metallic base plate; and a heating resistor arranged on a reverse face of the metallic base plate, the heating resistor being formed by sequentially laminating, at least, a heat transmission strengthening layer and a second temperature controlling type heating resistor layer on a set of a heat transmission strengthening layer and a first temperature controlling heating resistor layer, and further laminating thereon a heat insulation strengthening layer, heat reflecting layer and protective layer in this order, and the heat plate is curved so that the metallic base plate has a convex surface.
Furthermore, the present invention provides a multifunction fixing roller-inscribing belt type fixing device comprising a fixing roller arranged inside a semicircular heating member for fixation, an endless belt for fixation wound round an outer circumference of the semicircular heating member, and a pressure roller arranged on a face of the endless belt opposing to the fixing roller, in which the endless belt is heated to a predetermined temperature, and while a load is given between the fixing roller and the pressure roller, a recording sheet preliminarily heated by a heat plate for preliminarily heating is passed between the fixing roller and the pressure roller, thereby thermally fixing a toner image on the recording sheet, and
the semiconductor heating member is one member selected from the group consisting of the heating members (1) to (4) of the present invention described above.
Furthermore, the present invention provides an electrophotographic image forming apparatus comprising: an electrostatic latent image forming section for forming an electrostatic latent image by image exposure; a developing section for developing the electrostatic latent image with a developing agent comprising a toner to visualize the electrostatic latent image; an image transfer section for transferring the visualized toner image to a recording medium; and an image fixing section for fixing the transferred toner image to the recording medium, in which
the image fixing section comprises a multifunction fixing roller-circumscribing belt type fixing device comprising a pressure roller arranged inside a semicircular heating member for fixation, an endless belt for fixation wound round an outer circumference of the semicircular heating member, and a fixing roller arranged on a face of the endless belt opposing to the pressure roller, in which the endless belt is heated to a predetermined temperature, and while a load is acting between the pressure roller and the fixing roller, the recording medium preliminarily heated by a heat plate for preliminarily heating is passed between the pressure roller and the fixing roller, thereby thermally fixing the toner image on the recording medium, and
the semicircular heating member is one member selected from the group consisting of the heating members (1) to (4) of the present invention described above.
Moreover, the present invention provides an electrophotographic image forming apparatus comprising: an electrostatic latent image forming section for forming an electrostatic latent image by image exposure; a developing section for developing the electrostatic latent image with a developing agent comprising a toner to visualize the electrostatic latent image; an image transfer section for transferring the visualized toner image to a recording medium; and an image fixing section for fixing the transferred toner image to the recording medium, in which
the image fixing section comprises a multifunction fixing roller-inscribing belt type fixing device comprising a fixing roller arranged inside a semicircular heating member for fixation, an endless belt for fixation wound round an outer circumference of the semicircular heating member, and a pressure roller arranged on a face of the endless belt opposing the pressure roller, in which the endless belt is heated to a predetermined temperature, and while a load is acting between the fixing roller and the pressure roller, the recording medium preliminarily heated by a heat plate for preliminarily heating is passed between the pressure roller and the fixing roller, thereby thermally fixing the toner image on the recording medium, and
the semicircular heating member is one member selected from the group consisting of the heat members (1) to (4) of the present invention described above.